SYNTHESIS OF 3BaO-2MgO-2Nb 2 O 5 MICROWAVE DIELECTRIC CERAMICS BY A REACTION-SINTERING PROCESS Yi-Cheng Liou*, Wei-Ting Li Yi-Cheng Liou*, Wei-Ting Li Department of Electronics Engineering, Kun Shan University, Tainan Hsien 71003, Taiwan, R.O.C. *Corresponding author. Synthesis of 3BaO-2MgO-2Nb 2 O 5 (BMN) microwave dielectric ceramics by a reaction-sintering process was investigated in this study. Without any calcination involved, the mixture of raw materials was pressed and sintered directly. Ba(Mg 1/3 Nb 2/3 )O 3 phase formed as a major phase, and the minor phases Ba 3 Nb 5 O 15 and Ba 3 Nb 8 O 21 also formed at 1250  C/2 h sintering. Ba 3 Nb 5 O 15 phase decreased while Ba 3 Nb 8 O 21 phase increased as sintered at higher temperatures. Increased MgO content in (5-x)BaO-xMgO-2Nb 2 O 5 could lower the sintering temperature from 1450 o C (x=0) to 1250 o C (x=2). Some sub-micron (< 0.5 μm) MgO particles were segregated at the surfaces of grains. The reaction-sintering process has proven a simple and effective method in preparing 3BaO-2MgO-2Nb 2 O 5 microwave dielectric ceramics. Fig. 1 shows XRD patterns of BMN ceramics produced using the reaction- sintering process. Ba(Mg 1/3 Nb 2/3 )O 3 phase formed as a major phase, and the minor phases Ba 3 Nb 5 O 15 and Ba 3 Nb 8 O 21 also formed at 1250  C/2 h sintering. Ba 3 Nb 5 O 15 phase decreased while Ba 3 Nb 8 O 21 phase increased as sintered at higher temperatures. These are different from Ba 5 Nb 4 O 15 prepared using the reaction-sintering process. The mixture of BaCO 3 and Nb 2 O 5 was efficiently transformed into the Ba 5 Nb 4 O 15 phase even with the calcination stage bypassed. Therefore, (Ba 3 Mg 2 )Nb 4 O 15 ceramics with A 5 B 4 O 15 structure could not be prepared using the reaction-sintering process. On the contrary, Ba 3 (MgNb 2 )O 9 (i.e. Ba(Mg 1/3 Nb 2/3 )O 3 ) with ABO 3 structure formed much easier in the pellets. Since the ionic radius of Mg 2+ (0.078nm) is much smaller than that of Ba 2+ (0.143nm), it would substitute for Nb 5+ in stead of Ba 2+. (Ba 4.5 Mg 0.5 )Nb 4 O 15 phase formed as a major phase in (5-x)BaO-xMgO-2Nb 2 O 5 with x=0.5. While with x=1 the major phase Ba(Mg 1/3 Nb 2/3 )O 3 formed in our previous study. This implies only less than 10% Ba of the A-site was replaced by Mg in the A 5 B 4 O 15 structure. The shrinkage results for BMN ceramics are shown in Fig. 2. Values are ranged in 21-24% o C is high enough for densification. In Fig. 3, the density of BMN ceramics increased with the sintering temperature and saturated above 1270 o C. In our study of BaNb 4 O 15, the density increased with the sintering temperature and reached a maximum value 6.13g/cm 3 (97.3% of the theoretical value) at 1450  C for 4 h and 6 h. Density lower than 5g/cm 3 were found in (5- x)BaO-xMgO-2Nb 2 O 5 with x=0.5 even sintered at  C. With x=1, density exceeded 5g/cm 3 at 1330  C sintering. As CuO (1 wt%) was added, the sintering temperature dropped more than 150  C and density saturated at 1200  C. Increased MgO content in (5-x)BaO-xMgO-2Nb 2 O 5 could lower the sintering temperature from 1450 o C (x=0) to 1250 o C (x=2 in this study). SEM photographs of as-fired BMN ceramics sintered at o C for 2 h are shown in Fig. 4. Porous pellets were formed at 1250 o C sintering, which was in good agreement with the density values in Fig. 3. It can be easily observed that pores decreased as sintering temperature increased. In (5-x)BaO- xMgO-2Nb 2 O 5 with x=0.5, thin and flat (Ba 4.5 Mg 0.5 )Nb 4 O 15 grains 10 μm wide clearly visible. Round Ba(Mg 1/3 Nb 2/3 )O 3 grains became the major morphology in pellets with x=1. For x=2 in this study, grains of different morphology were observed. Larger grains in Fig. 4 are Ba(Mg 1/3 Nb 2/3 )O 3 and this matched our finding that Ba(Mg 1/3 Nb 2/3 )O 3 was the major phase, as shown in the XRD of Fig. 1. Smaller grains (< 2 μm) in pellets sintered at 1250 o C are Ba 3 Nb 5 O 15 and Ba 3 Nb 8 O 21. The amount of Ba 3 Nb 8 O 21 increased at higher sintering temperatures. It is also noted that some sub-micron (< 0.5 μm) particles were segregated at the surfaces of grains. These are thought to be MgO particles. The reason caused the MgO segregation was: only one phase (Ba(Mg 1/3 Nb 2/3 )O 3 or Ba 3 (MgNb 2 )O 9 ) formed with the participation of MgO, no participation of MgO in the formation of Ba 3 Nb 5 O 15 and Ba 3 Nb 8 O 21. Therefore, the unreacted MgO segregated. More MgO particles segregated at 1300 o C and 1330 o C due to the high Nb contained phase Ba 3 Nb 8 O 21 increased which lead to more unreacted MgO remained in the pellets. In (5-x)BaO-xMgO-2Nb 2 O 5 with x=0.5 and 1, there was no MgO segregation in the microstructure morphology. Fig. 1 XRD patterns of BMN ceramics sintered at (A) 1250 o C, (B) 1270 o C, (C) 1300 o C, and (D) 1330 o C for 2 h. (1) Ba(Mg Nb )O 3 : ICDD PDF # , (2) Ba 3 Nb 8 O 21 : JCPD # , and (3) Ba 3 Nb 5 O 15 : JCPD # Fig. 4 SEM photographs of as-fired BMN ceramics sintered at (A) 1250 o C, (B) 1270 o C, (C) 1300 o C, and (D) 1330 o C for 2 h. Fig. 3 Density of BMN ceramics sintered at various temperatures and soak time. Fig. 2 Shrinkage percentage of BMN ceramics sintered at various temperatures and soak time. Materials and Austceram 2007 July 4 - 6, 2007, Sydney, Australia